Acrylic resins have enjoyed extensive use as coating materials for many substrates in recent years. The acrylic resins are useful as they are capable of forming good hard films which are quite protective to the substrate on which they are used.
A problem involving the use of acrylic resin coatings has been the expense and time involved in curing the resins to a hard coating. The use of catalysts and heating in an oven is a slow and expensive process.
The use of ionizing irradiation as a fast and comparatively inexpensive method of curing selected compositions is now known. Coating compositions containing 3-acrylyloxy-2,2-dimethylpropyl 3-acrylyloxy-2,2-dimethylpropionate (viz., acryloxypivalyl acryloxypivalate), 1,3-bis(acrylyloxy)-2,2-dimethylpropane (viz., neopentyl glycol diacrylate) or mixtures thereof and further containing as hiding pigment titanium dioxide, zinc oxide, lithopone or mixtures thereof produce especially fine coatings when cured in a substantially inert curing atmosphere using ionizing irradiation. The curing of these coating compositions is quite sensitive to inhibition by oxygen. When coatings of these materials are exposed to ionizing irradiation while in contact with a curing atmosphere containing a cure inhibiting amount of oxygen (usually about 100 parts by volume oxygen per million parts by volume curing atmosphere or greater), the surfaces are observed to exhibit an unsatisfactory mar resistance. Exposure to even very large amounts of ionizing irradiation does not produce satisfactory surface mar resistance under these conditions.
The maintenance of oxygen concentrations as low as 100 parts per million is both difficult and expensive. Even small amounts of air entering the system through leaks or, more usually, by being entrained by the coated substrates passing through the curing apparatus, can easily raise the oxygen concentration above acceptable limits unless inordinate amounts of an inert gas such as nitrogen, carbon dioxide, argon, helium, methane, ethane or propane are used to purge the system. For example, in a system where ideal mixing can be assumed, it is necessary to purge with about 2100 volumes of pure inert gas per unit time for each volume of air per unit time entering the system in order to maintain an oxygen concentration of 100 parts per million parts, by volume. Should the volumetric flow rate of air be appreciable, the volumetric flow rate of gas passing through the apparatus is seen to be enormous. If the acceptable level of oxygen concentration of such a system can be raised to 300 parts per million, only about 700 volumes of pure inert gas per volume of air per unit time are required to maintain the oxygen concentration at 300 parts per million. If the acceptable level of oxygen concentration of the system can be raised to 1000 parts per million, the ratio of volumetric flow rates is reduced to about 209. If the inert gas contains oxygen, these ratios are even larger. Although ideal mixing is not strictly attainable in any real system, the assumption is useful in estimating, to a reasonable degree of accuracy, the approximate inert gas requirements of a real system.
It has now been discovered that triphenyl phosphine may be admixed with either or both of the diacrylate monomers and one or more of the hiding pigments hereinbefore described to form a composition which cures rapidly when subjected to ionizing irradiation in the presence of a curing atmosphere containing from about 300 to about 1000 parts per million oxygen. A hard film having a high degree of surface mar resistance is formed by exposing a coating of an ionizing irradiation curable coating composition comprising (1) diacrylate monomer which is 3-acrylyloxy-2,2-dimethylpropyl 3-acrylyloxy-2,2-dimethylpropionate, 1,3-bis(acrylyloxy)-2,2-dimethylpropane or mixtures thereof, (2) hiding pigment which is titanium dioxide, zinc oxide, lithopone or mixtures thereof, and (3) from about 0.5 to about 10 percent triphenyl phosphine by weight of the composition while the surface of the coating opposite the substrate upon which it is coated is in contact with a curing atmosphere containing from about 300 to about 1000 parts oxygen by volume per million parts curing atmosphere by volume, to at least about 4 megarads of ionizing irradiation of an energy equivalent to at least 100,000 electron volts to cure the coating.
Although the preferred curing atmosphere contains only slightly greater than 300 parts per million of oxygen, the use of a greater amount of triphenyl phosphine allows the irradiation to take place in a curing atmosphere containing a greater amount of oxygen. Generally, the curing atmosphere contains from about 300 to about 500 parts oxygen by volume per million parts curing atmosphere by volume.
The compounds 3-acrylyloxy-2,2-dimethylpropyl 3-acrylyloxy-2,2-dimethylpropionate and 1,3-bis(acrylyloxy)-2,2-dimethylpropane and methods for their preparation are known to the art. Preparation of the former compound is described in Example 1 of U.S. Pat. No. 3,645,984. Preparation of the latter is shown in British Patent Specification No. 1,067,433.
It is noted that the compounds and mixtures of this invention may be mixed with other monomers and polymers. Examples of polymers which may be utilized along with the novel mixtures of this invention are acrylic polymers, vinyl polymers, and polyesters. Examples of other monomers which may be used along with the mixtures are acrylic monomers such as alkyl acrylates and methacrylates such as ethyl acrylate, butyl acrylate, 2-ethylhexylacrylate and methacrylate, lauryl acrylate and methacrylate and hydroxy alkyl acrylates and methacrylates such as hydroxyethyl acrylates and hydroxy methylmethacrylate and the like. Other materials may also be used to enhance the physical properties of the coating composition such as conventional pigments, plasticizers, etc.
The coating compositions may be prepared by simply admixing the various ingredients.
The diacrylate monomer described above generally constitutes from about 30 percent to 100 percent by weight of the binder of the coating composition. Typically, the diacrylate monomer is present in an amount in the range of from about 70 percent to 100 percent by weight of the binder.
Hiding pigment employed in the coating compositions of the invention is titanium dioxide, zinc oxide, lithopone or mixtures thereof. The hiding pigment to diacrylate monomer weight ratio of the coating compositions is usually in the range of from about 0.3:1 to about 3:1 where the diacrylate monomer is that heretofore described. Tinting pigments, extending pigments, other hiding pigments and/or dyes may, if desired, additionally be present. The preferred hiding pigment is titanium dioxide. The titanium dioxide may be rutile, anatase or mixtures thereof. Rutile is especially preferred.
The triphenyl phosphine may be admixed with the diacrylate monomer at any time prior to the ionizing irradiation treatment. The triphenyl phosphine is believed to act as an oxygen scavenger in the irradiation treatment. The triphenyl phosphine may be present in the amount of 0.5 to about 10 percent or greater by weight of the composition. With the addition of triphenyl phosphine, the irradiation can take place in an atmosphere containing 1000 parts per million of oxygen.
The term "irradiation," as used herein, means high energy radiation and/or the secondary energies resulting from conversion of electrons or other particle energy to X-rays or gamma radiation. While various types of irradiation are suitable for this purpose, such as X-ray and gamma rays, the radiation produced by accelerated high energy electrons has been found to be very conveniently and economically applicable and to give very satisfactory results. However, regardless of the type of radiation and the type of equipment used for its generation or application, the use thereof in the practice of the invention as described herein is contemplated as falling within the scope of this invention so long as the ionization radiation is equivalent to at least about 100,000 electron volts.
While there is no upper limit to the electron energy that can be so applied advantageously, the effects desired in the practice of this invention can be accomplished without having to go to above about 20 million electron volts. Generally, the higher the electron energy used, the greater is the depth of penetration into the massive structure of the materials to be treated. For other types of radiation, such as gamma and X-rays, energy systems equivalent to the above range of electron volts are desirable.
It is intended that the term "irradiation" include which has been referred to in the prior art as "ionizing radiation" which has been defined as radiation possessing an energy at least sufficient to produce ions or to break chemical bonds and thus includes also radiations such as "ionizing particle radiation" as well as radiations of the type termed "ionizing electromagnetic radiation."
The term "ionizing particle radiation" has been used to designate the emission of electrons or highly accelerated nuclear particles such as protons, neutrons, alpha-particles, deuterons, beta-particles, or their analogs, directed in such a way that the particle is projected into the mass to be irradiated. Charged particles can be accelerated by the aid of voltage gradients by such devices as accelerators with resonance chambers, Van der Graaff generators, betatrons, synchrotrons, cyclotrons, etc. Neutron radiation can be produced by bombarding a selected light metal such as beryllium with positive particles of high energy. Particle radiation can also be obtained by the use of an atomic pile, radioactive isotopes or other natural or synthetic radioactive materials.
"Ionizing electromagnetic irradiation" is produced when a metallic target, such as tungsten, is bombarded with electrons of suitable energy. This energy is conferred to the electrons by potential accelerators of over 0.1 million electron volts (mev.). In addition to irradiation of this type, commonly called X-ray, an ionizing electromagnetic irradiation suitable for the practice of this invention can be obtained by means of a nuclear reactor (pile) or by the use of natural or synthetic radioactive material, for example, cobalt 60.
Various types of high power electron linear accelerators are commercially available, for example, the ARCO type travelling wave accelerator, model Mark I, operating at 3 to 10 million electron volts, such as supplied by High Voltage Engineering Corporation, Burlington, Mass., or other types of accelerators as described in U.S. Pat. No. 2,763,609 and in British Pat. No. 762,953 are satisfactory for the practice of this invention.
The diacrylate monomer combined with triphenyl phosphine will cure acceptably in the presence of 300 parts per million of oxygen or more using any total dosage greater than about 4 megarads. A "rad" is defined as that amount of radiation required to supply 100 ergs per gram of material being treated, and a "megarad" is 10.sup.6 rads. The total dosage is the total amount of irradiation received by the coating compositions. It is noted that the same compositions without the triphenyl phosphine could not be cured in the presence of a substantial amount of oxygen even at 100 megarads and higher to produce surfaces having satisfactory mar resistances.
The novel method of this invention may be used to coat substrates with the diacrylate monomer by merely applying the composition to the substrate and subjecting the composition to ionizing irradiation to cure in situ.
Any conventional means of applying the composition to the substrate may be used such as dip coating, roll coating, spraying and the like.
The coated substrates are quite useful for plywood paneling, cabinets, furniture, pringed paper products, cement, and cement asbestos products, and the like.